1. Field of the Invention
The present invention relates generally to a power amplification apparatus applied to a wireless communication system, and in particular, to a Doherty amplifier obtained by improving gain and linearity of a power amplification apparatus for a base station.
2. Description of the Related Art
Generally, the only factor that has been considered important with respect to a base station (BS) in a wireless communication system is linearity. This is because a power amplification apparatus for a base station has sufficient power. Recently, however, because such factors as output improvement, miniaturization and operation cost reduction for the base station are regarded as important factors, research is now being conducted on improving efficiency of the power amplification apparatus.
A Doherty amplifier has been proposed to improve efficiency of the power amplification apparatus. Commonly, the Doherty amplifier has a structure in which a carrier amplifier is asymmetrically combined in parallel with a peaking amplifier.
The Doherty amplifier, a type of amplifier used for high-performance modulation in a high-power transmitter such as a base station, was first proposed by W. H. Doherty in 1936, and has a structure in which a carrier amplifier is connected in parallel to a peaking amplifier using a quarter-wave transformer (or λ/4 line). The peaking amplifier in the Doherty amplifier adjusts impedance of a load line of the carrier amplifier through a method of providing a varying amount of current to the load according to a power level, thereby increasing an efficiency characteristic of the Doherty amplifier.
FIG. 1 is a block diagram schematically illustrating a structure of a general Doherty amplifier used in a wireless communication system.
Referring to FIG. 1, the Doherty amplifier includes a divider 101, a carrier amplifier 103, a peaking amplifier 105, and transformers 107 and 109. The Doherty amplifier has a structure in which the carrier amplifier 103 is asymmetrically combined in parallel with the peaking amplifier 105.
In the Doherty amplifier, the peaking amplifier 105 performs a varying operation according to its input power. That is, for low input power, the peaking amplifier 105 does not perform amplification operation on the input power, and is turned OFF. However, for high input power, the peaking amplifier 105 performs amplification operation on the input power, and is turned ON. In other words, for the low input power, the carrier amplifier increases its gain such that the load seen at the carrier amplifier 103 is two times higher than that for the high input power, thereby showing higher efficiency compared with that of a symmetrically parallel-combined amplifier, such as a hybrid combined amplifier.
An operation of the Doherty amplifier shown in FIG. 1 will now be described below.
An output network including combining units, i.e., the transformers 107 and 109, of the Doherty amplifier is implemented by connecting a transmission line 107 with a characteristic impedance Zo and a length λ/4 to an output of the carrier amplifier 103 and connecting a Zo/√{square root over (2)} load 109 to a rear end of the transmission line 107. An output end of the peaking amplifier 105 is connected to a contact point between the transmission line 107 and the Zo/√{square root over (2)} load 109. If low input power is applied to this structure, the peaking amplifier 105 is turned OFF, so load impedance seen at the carrier amplifier 103 doubles to 2·Zo. However, if high input power is applied to the Doherty amplifier, the peaking amplifier 105 is turned ON, so impedance after the λ/4 transmission line becomes Zo due to an output current of the peaking amplifier 105. As a result, impedance seen at the carrier amplifier 103 becomes Zo, and the Doherty amplifier is driven at its maximum output power.
The phenomenon in which impedance seen at the carrier amplifier 103 changes, i.e., gain or output power suffers change, is called “load modulation”. The Doherty amplifier, compared with the hybrid combined amplifier, increases in efficiency due to the load modulation operation.
If the conventional Doherty amplifier receives the low input power as described above, the carrier amplifier 103 doubles in gain due to the load modulation operation. As a result, the total gain remains unchanged even though the peaking amplifier 105 operates.
However, unlike an ideal Doherty amplifier the conventional actual used Doherty amplifier suffers power leakage toward the peaking amplifier 105. Therefore, its actual gain decreases due to the leaked power, the imperfect load modulation operation and an insertion loss of the divider 101.
In addition, the conventional Doherty amplifier further suffers linearity degradation due to non-linear operation of the peaking amplifier 105. Moreover, the Doherty amplifier cannot be designed such that the peaking amplifier 105 is correctly turned ON/OFF according to a power level, making it impossible to acquire its expected efficiency.
Accordingly, there is a need for an improved Doherty amplifier design scheme capable of increasing gain and linearity by solving the problems of the conventional Doherty amplifier.